It is known that ceramic films or membranes may be made of metal oxide materials, such as titanium dioxide. Such membranes are typically made by a sol-gel process in which a metal oxide precursor, typically an organometallic compound such as a metal alkoxide, is dissolved in an alcohol at low temperature and hydrolyzed and peptized to create a colloidal suspension or sol. Such sols can be slowly dewatered or can be coated onto substrates to form gels, which can be sintered into ceramic membranes, either unsupported or supported. By controlling the conditions of the sol-gel process, the metal oxide can be manipulated to form particles of selected size, which when fused into a particulate membrane, results in a membrane of a selected average hole or pore size.
Normally in such membranes, porosity and conductivity are inversely related. This relation would be expected since increasing porosity is related to less contact between the sintered particles, and thus less surface area for electron flow. This property can, however, be a disadvantage in any application in which both high porosity and high conductivity are desirable.
One application in which both high porosity and high conductivity are desirable is the use of such a metal ceramic membrane as an electrode in an electrochemical cell. In such an application, it is desirable to have maximum surface area contact between the electrodes and the ionic solution of the cell. Higher porosity gives a higher effective surface area between the membrane used as an electrode and the solution into which it is placed. At the same time, it is clear that high conductivity is desirable for any material to be used as an electrode, to facilitate current flow into and out of the electrode from the appropriate electric circuitry.
Porosity and heat stability are also usually inversely related in metal oxide ceramic membranes. We have reported that microporous TiO.sub.2 and ZrO.sub.2 membranes have been successfully synthesized in our laboratory. However, the thermal stability of these membranes has been seriously challenged. These single component membranes are only able to withstand a temperature of below 350.degree. C. in order to avoid closure of the micropores. In a general sense, ceramic materials formed by this low-temperature processing are not mechanically strong enough for many uses. It is desirable for such membranes to be as thermally stable as possible, in order to be useful in high-temperature applications.
Ceramic membranes prepared by sol-gel method are an assembly of individual particles, either crystalline or amorphous, containing pores. During the firing process, a number of chemical and physical changes in the membrane and its pores take place due to the applied heat. According to Reed (Introduction to the Principles of Ceramic Processing, p. 440, ed: Wiley & Sons, New York, 1988) the firing process may proceed in three stages: (1) presintering, in which the temperature is lower than one-half the melting point of a specific oxide; (2) sintering; and (3) cooling. Many chemical reactions and physical changes occur during the first presintering process. These reactions and changes include evaporation of liquids (which can be either contained in the pores or absorbed on the particle surfaces); burnout of binders and other organics associated with particles; decomposition of volatile inorganic acids, salts and other compounds which are used as additives in the preparative processing; and phase transitions which, among solid phases, refer to crystallization or transition between two crystalline phases. Sintering is a process associated with a decrease of surface energy by loss of surface area, usually with accompanying densification. Sintering, according to some authors, does not commonly begin until the temperature reaches a point where significant atomic diffusion within the material can occur. This temperature is reported to occur at the one-half to two-thirds of the melting point.
Microstructural changes in sintering process have been well-studied in past decades. Increase in particle size and decrease in porosity are two major features of changing microstructure of ceramic material during sintering. However, the microstructural variations in the presintering stage have not been well investigated. This is likely because of the difficulty of this research which arises from too many complicated reactions taking place simultaneously during this firing stage. In addition, microstructural variation with temperature largely depends on the nature of the individual material and its preparative history.
In contrast with completely sintered dense ceramic material, ceramic membranes are porous materials. These materials can only be fired at low temperatures, i.e. temperatures high enough to form a coherent body but no so high as to eliminate pores. Therefore, studies on the microstructural change in ceramic membranes during the presintering process, which are usually ignored by many ceramists, become important. The micropores in TiO.sub.2 or ZrO.sub.2 membranes prepared by sol-gel method seem to disappear by mechanisms associated with crystallization during the presintering process.
What is needed in the art of ceramic membranes formation is a metal oxide ceramic membrane of small pore size and high surface area with increased thermal stability.